In general, an industrial-use gas burner has been known such as a configuration whose flame is formed in front of the tip of a burner. Concerning such a burner, fuel supplied through a fuel-passage and combustion air supplied through an air-passage are sprayed in front of the burner from the nozzle, resulting in forming the turbulence by the sprayed air and fuel.
Accordingly, the combustion flame becomes turbulent, and the partial flame extinction happens. Such partial flame extinction makes the combustion not stable. In order to avoid such a phenomenon as much as possible, nozzle is designed to exhibit the optimal nozzle-flow-velocity so that stable combustion is obtained, which corresponds to the particular heating value and combustion speed of the employed fuel from the thermal perspective and the perspective of fluid dynamics.
In such a case, the stable combustion is done when using the fuel suitable for the designed nozzle. On the other hand, combustion becomes unstable when using other kinds of fuel.
Furthermore, combustion reaction is always performed within a flame that has a certain volume, so the reaction is required to continue for a long period. In such a case, NOx or soot is apt to generate by the reason of the long combustion time. And, the flame has a partial high-temperature region and a low-temperature region, wherein NOx is easy to generate in the high-temperature region, and soot is easy to generate in the low-temperature region.
On the other hand, a tubular flame burner is disclosed in Japanese Unexamined Patent Application Publication No. 11-281015. This publication includes a tubular combustion chamber of which one-end opens and a nozzle for spraying a fuel gas and a nozzle for spraying an oxygen-containing-gas in the neighborhood of the closed end thereof. Here, the nozzle is located, facing in the tangential direction of the inner circumferential wall of the aforementioned combustion chamber.
With the aforementioned tubular flame burner, stable flame is formed in a high-speed swirl within the burner, accordingly combustion is performed with small irregularities in the temperature of a combustion flame. Therefore, no partial high-temperature regions are easy to be formed. Furthermore, stable combustion is achieved even with a low oxygen ratio or air excess ratio. Consequently, the tubular flame burner has the advantage to reduce harmful substances such as NOx or the like, unburned portions of hydrocarbon or the like, and environmental pollutants such as soot and the like, as well as to reduce of the size thereof.
FIG. 8 is explanatory diagrams which show an conventional tubular flame burner, wherein FIG. 8A is a configuration diagram which shows the tubular flame burner, and FIG. 8B is a cross-sectional view taken along line B-B in FIG. 8A. The tubular flame burner includes a tubular combustion chamber 121, whose one end opens for serving as an exhaust vent for an exhaust gas. Furthermore, the tubular flame burner includes long slits on the other end along the tube axis, each of which are connected to one of nozzles 122 for separately supplying a fuel gas and a nozzle for supplying an oxygen-containing-gas.
The nozzles 122 are disposed in a tangential direction of the inner wall of the combustion chamber 121 for spraying the fuel gas and the oxygen-containing-gas so as to form a swirl thereof within the combustion chamber 121. Furthermore, the tip of each nozzle 122 is formed flat with a reduced orifice for spraying the fuel gas and the oxygen-containing-gas at high speed. Note that reference numeral 123 denotes a spark plug.
In the above-mentioned burner having such a configuration, when a mixture gas is ignited, which forms a swirl (such a swirl is generated by the fuel gas and the oxygen-containing-gas sprayed from the nozzles 122), the gas within the combustion chamber 121 is stratified into concentric gas layers with different densities, due to difference in the density of the gas and the centrifugal force. That is to say, a high-temperature and low-density exhaust gas exists close to the axis of the combustion chamber 121, and a high-density unburned gas exists close to the inner wall of the combustion chamber 121 (away from the axis thereof). This state exhibits remarkable stability from the viewpoint of fluid dynamics. In this case, a tube-shaped flame is formed, and the gas flow is stratified into stable layers, thereby forming a film-shaped stable flame. The position of the flame is determined, being influenced by the position, wherein two factors (one is the exhaust gas speed toward the center of the combustion chamber 121 and the other is the flame propagation speed) balance each other in natural process. In FIG. 8A, reference numeral 124 denotes a tube-shaped flame.
Furthermore, an unburned low-temperature gas forms a boundary layer near the inner wall of the combustion chamber. Accordingly, the wall of the combustion chamber 121 is not heated by the direct heat transfer to a degree of a high temperature, resulting in avoiding the thermal loss, which means, preventing the heat from releasing to the outside of the wall. That is to say, the aforementioned burner has the effective advantage on great thermal insulation, thereby maintaining thermal stability of combustion.
The gas within the combustion chamber 121 flows downstream while swirling, and at the same time, the mixture gas around the inner wall continuously burns so as to form a tubular flame. And, a generated exhaust gas flows toward the axis of the combustion chamber 121 so as to be discharged from the open-end.
However, the conventional tubular flame burner having such a configuration happens to have problems as follows. That is to say:
In general, a fuel gas that has a small heating value invites a disadvantage, that is, the range of the air excess ratio is extremely narrow, taking into consideration the usable range for igniting by electronic spark. Therefore, it is extremely difficult to ignite such a fuel without premixing of the fuel gas and the oxygen-containing-gas.
The aforementioned tubular flame burner has the same difficult problem on igniting by the electronic spark due to the limited range of the air excess ratio of the fuel gas and the oxygen-containing-gas suitable for the ignition. Accordingly, it may be a case, the aforementioned tubular flame burner requires a pilot burner.
Furthermore, the conventional tubular flame burner has such problems as the following description.    (1) In particular, in case of using oil fuel or heavy-hydrocarbon fuel such as a propane gas, the free carbon content within the fuel emits light during combustion, resulting in forming a luminous flame. The luminous flame has such a characteristic that the radiation rate is high by himself, resulting in increasing radiation heat from the luminous flame. Accordingly, when the burner having a configuration, whose luminous flame is located in the position capable of viewing from the heated material, the aforementioned burner exhibits high heat transfer efficiency. However, with the aforementioned conventional burner, the fuel sprayed into the furnace does not form a luminous flame, but forms a transparent exhaust gas that has small emissivity due to the complete combustion of the fuel within the combustion chamber. This leads to small heat transfer efficiency of the combustion method with the conventional tubular burner.    (2) With the conventional tubular burner, no soot is generated due to complete combustion of the fuel. Accordingly, the conventional tubular burner is not used in case of requiring soot, for example, such as carburizing steel with high efficiency, for example.    (3) The conventional tubular burner exhibits excellent combustion performance due to complete combustion of the fuel within the combustion chamber, but NOx is easy to be generated.
Furthermore, the conventional tubular flame burner has a configuration, wherein, in order to form a tubular flame, the respective supply nozzles that are flat along the tube axis are connected to the slits extending along the tube axis. (The slits are located in the tubular combustion chamber.) The conventional tubular flame burner is used while spraying the fuel gas and the oxygen-containing-gas into the combustion chamber, simultaneously with forming high-speed swirl of the sprayed fuel gas and the oxygen-containing-gas. Accordingly, the conventional tubular flame burner causes such a problem that relatively high pressure loss happens at the slits. That is to say, in general, the fuel gas and the oxygen-containing-gas are supplied with a constant pressure. Accordingly, there is need to increase the flow of the fuel gas and the oxygen-containing-gas, in case of increasing the combustion load. But in this case, the pressure loss at the slits increases, proportional to the squire value of the flow speed, ending up in a small increase in a combustion load.
Contrarily, when the conventional tubular flame burner having a configuration is used (wherein each slit is formed with an increased cross-sectional area so as to reduce the pressure loss at the slit), the flow speed of the fuel gas and the oxygen-containing-gas remarkably reduce along the tangential direction of the inner wall of the combustion chamber. Such reduction happens in the event that combustion is performed with a small flow of the fuel gas and the oxygen-containing-gas corresponding to a small combustion load. Accordingly, a tube-shaped flame is not formed, leading to such a problem as increased amount of NOx, soot, and the like, generated in the combustion chamber.
As described above, concerning the conventional tubular flame burner, the problem is as follows. In the event that the supply flow of the fuel gas and the oxygen-containing-gas is adjusted corresponding to the change in the combustion load, it may be a case, the flow speed of the fuel gas and the oxygen-containing-gas is out of the range of the suitable flow speed. The suitable flow speed is determined between the flame formation minimal flow speed required for formation of a tube-shaped flame and the permissive maximal flow speed dependent upon the pressure loss, inviting difficulty in stable combustion in a wide range of the combustion load, and resulting in a narrow range of the combustion load suitable for the conventional tubular flame burner.
Furthermore, there is need to further improve the aforementioned conventional tubular flame burner in order to employ fuel with lower heat output so as to improve the practical use.
Accordingly, the present invention has been conceived in order to solve the aforementioned problems of the conventional tubular flame burner. And the present invention has been conceived and studied in order to provide a tubular flame burner having a new flame formation mechanism, wherein various kinds of fuel can be used, wherein combustion is performed in a wide combustion range, and wherein stable combustion is maintained even with a wide range of the change in combustion load. And in the present invention, stable combustion can be performed, and discharge of an environmental pollution substance created due to combustion is prevented.